The present invention relates to borehole logging tools and particularly to a tool that is described as a borehole televiewer (BHTV). A borehole televiewer utilizes a rotating acoustic transducer that is periodically pulsed to produce acoustic energy which travels outwardly and strikes the borehole wall and is reflected back to the transducer. The amplitude of the reflected signal is displayed on a cathode ray tube whose face may be photographed. The resulting photograph represents a map of the borehole wall split along the north direction and laid out flat. The transducer is pulsed at a relatively high rate, for example, 1500 pulses per second, while the transducer rotation period is 0.333 seconds. The sweep of the oscilloscope is controlled by a north synchronization signal which is produced by a flux gate magnetometer which rotates in unison with the transducer.
While the above tools are useful in obtaining a display of the borehole wall that indicates the locations of fractures or other features, it does have a a serious limitation when used in a non-circular or elliptical borehole. The same problem also arises when the tool moves off center as a result of its travel through the borehole. It can be shown that when the BHTV is located off center or in a non-circular hole, the acoustic energy will be scattered at all locations except where the acoustic energy strikes the borehole wall normal to a tangent at this point. Thus, the reflected energy received by the transducer will have four positions where it is at a maximum and in the remaining positions the reflected energy will be decreased. The BHTV as presently available has various amplifier gain levels that are selected by a rotary switch controlled from the surface. Since the amplitude is set from the surface, it cannot be varied to provide the proper amplification as the transducer rotates. Thus, the resulting display will have light and dark areas where details are lacking. It should be appreciated that in normal operations a fracture or other anomaly in the borehole wall will not reflect energy and will appear as a dark shadow on the display.